The present invention relates generally to a strut suspension structure for an automotive vehicle, the geometry of which adjusts to vehicle driving conditions so as to prevent suspension camber from changing to positive camber, to lower the suspension harshness level and to suppress vehicular pitch during acceleration and deceleration. More specifically, the invention relates to a mounting structure of a suspension strut assembly which allows the upper end of the suspension strut assembly to move in a direction perpendicular to the longitudinal axis of the strut assembly.
FIGS. 9 and 10 show a typical suspension geometry used in strut-type vehicular suspensions. A steering knuckle 107 supporting a road wheel 109 is supported in turn by a transverse link 103. The inner end of the transverse link 103 is connected to the vehicle body through an elastic bushing 101. On the other hand, the outer end of the transverse link 103 is connected to the aforementioned steering knuckle 107 via a lower ball joint 105. The steering knuckle 107 is also connected to the lower end of a suspension strut 111. The upper end 113 of the suspension strut 111 is connected to a vehicle body (not shown) via an elastic or resilient suspension support (not shown).
In such strut-type suspensions, in order to avoid significant change of wheel alignment during compliance steering or bump steering the suspension is aligned so that the longitudinal axis of the strut assembly 111 is approximately aligned with the deformation axis of the suspension support in order to constrain displacement of the upper end of the strut assembly to its longitudinal axis. This geometry effectively minimizes compliance or bump steering in response to minimal changes in wheel alignment.
However, this conventional geometry induces significant camber change toward positive camber in response to roll steering. Centrifugal forces on the vehicle during cornering tend to displace the outside wheels upwards. In such cases, the transverse link 103 pivots upwards as shown in phantom line in FIG. 9. This pivotal movement of the transverse link 103 causes displacement of the lower ball joint 105 not only upward but also transversely inward. Rolling of the vehicle body furthers this transverse displacement of the lower ball joint. As a result, the camber angle .theta. changes to positive-camber whereby the cornering force may be reduced far enough to allow transverse sliding of the vehicular wheels.
On the other hand, during bump steering, the wheel axis Wc tends to be displaced frontward, which increases the harshness level. The harshness level can be lowered by reducing the caster angle, whereby the center of inertia Pc of the suspension shifts inwards and so increases from r.sub.1 to r.sub.2 the lever arm working against a counter force R which is the vector sum of a vertical counter force R.sub.y and a load-shifting counter force R.sub.z. Explansion of the lever arm from r.sub.1 to r.sub.2 tends to exacerbate pitch during acceleration and deceleration, such as during braking. Thus, it has been considered impossible to achieve both of suppression of harshness and anti-dive characteristics.
It has been known that increased caster trail results in better centering characteristics. Greater caster trail ensures weak under-steer for good driving stability. However, this, in turn, tends to increase the required steering force.